Ps to cs handover indicator

ABSTRACT

The present invention relates to a solution for handling handover of connections for a user equipment from a packet switch network to a circuit switched network. This is provided as a method implemented in a node and system. The method is provided in a telecommunications network connecting user equipment, i.e. UE, communicating wirelessly with the network. The method comprising steps of detecting that handover is required from a packet switched, i.e. PS, based network to a circuit switched, i.e. CS, based network, initiating in a source mobility management node a single radio voice call continuity, i.e. SVRCC, initiating a PC to CS handover for non-voice components with information about voice related parameters and a PS to CS handover indicator, executing hand over, sending an update request to a serving gateway, i.e. SGW, from a target mobility management node with non-voice related parameters and the PS to CS handover indicator, forwarding the update request from the SGW to a packet data network gateway, i.e. PGW, receiving the update request in the PGW, detecting the PS to CS handover indicator, and handling in the PGW the PS to CS handover indicator.

This application is a Continuation of U.S. patent application Ser. No.14/163,278, filed Jan. 24, 2014, which is a Continuation of U.S. patentapplication Ser. No. 13/264,790, filed Oct. 17, 2011, now U.S. Pat. No.8,675,607, which is a 35 USC 371 National Phase application ofPCT/EP2010/055211 filed Apr. 20, 2010, which claims the benefit of U.S.provisional patent application No. 61/171,918, filed Apr. 23, 2009, thedisclosures of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a solution for handling handover ofconnections for a user equipment from a packet switch network to acircuit switched network.

BACKGROUND

In evolved Universal terrestrial radio access network (EUTRAN) both realtime IP multimedia Subsystem (IMS) and non-real time services such asInternet access may be provided using the Evolved Packet Core (EPC).EUTRAN coverage will initially be limited and thus existing voice dataand data calls must be possible to hand over to e.g. UTRAN and GERAN.UTRAN/GERAN network deployments usually rely on legacy a circuitswitched (CS) Core Network for handling voice calls while e.g. non-realtime data services may be provided over a GPRS Core network. Thedeployment of parallel CS infrastructure in the market to support UTRANand GERAN voice calls is expected to continue to exist for a long time.

To be able to handle this scenario an optional functionality calledSingle Radio Voice Call Continuity (SRVCC) has been defined in 3GPPrelease 8 in TS 23.2016 and TS 23.216.

TS 23.216 describes stage 2 functionality for SRVCC between EUTRANaccess and 3GPP 1×CS and between EUTRAN access and 3GPP UTRAN/GERANaccesses and between UTRAN (HSPA) access and 3GPP UTRAN/GERAN accessesfor circuit switched calls that are anchored in the IMS.

GPRS includes a preservation function, which allows an active PDPcontext associated with released Radio access bearers (RAB) to bepreserved in the core network (CN) when a UE goes out of coverage. TheRABs can then be re-established at a later stage when a UE comes backinto coverage. PDP contexts associated with a guaranteed bit rate (GBR)Quality of Service (QoS) are preserved when the maximum bit rate (MBR)for uplink and downlink is set to 0 kbit/s.

When SRVCC is performed from EUTRAN to UTRAN/GERAN with dual transfermode (DTM) packets switched (PS) handover support, as described in TS23.216 section 6.2.2.2, there exists a potential race condition that mayresult in the voice call being dropped prematurely. A mobilitymanagement entity (MME) differentiates or “splits” the voice bearersfrom all other PS bearers and initiates their relocation towards a MSCserver and an SGSN. After some time the MSC server, or MGW, initiates asession transfer towards IP multimedia Subsystem (IMS), the IMS, or aP-CSCF, will update service session information in a PCRF by removingaffected media components using an Rx interface—step “A”.

In parallel to this procedure, the MSC server/MGW responds to thehandover request to the source MME, which in turn initiates a handovercommand to the UE. The UE then tunes into UTRAN/GERAN and the handoveris completed. Finally, an S4-SGSN updates bearers with a serving gateway(SGW). The SGW forwards the request to a Packet data network gateway(PGW). Since the voice bearers can not be established by the S4-SGSN theS4-SGSN will release those bearers by triggering a bearer contextde-activation towards the SGW and PGW. When the PGW receives anindication that the voice bearers have been de-activated it will reportthis to the PCRF over a Gx interface—step “B”.

The PCRF will now be able to match the information received over Rx withthe information over Gx and take the necessary action, e.g. to removethe installed PCC rule in the PGW.

However, in these procedures step “B” may sometimes occur before step“A”. If this is the case the PCRF will have to assume that the voicebearer has been lost and report this to the P-CSCF. TS23.228 describesthat the P-CSCF may actually initiate release of the whole IMS sessionin case of a bearer related to the IMS is lost.

The preservation function as described in TS 23.060 us not supported inEUTRAN EPS, but it could be added to EPS as well; however, then thewhole QoS information element (IE) would have to be sent when indicatingthat the bearer has been preserved—HO to PS. Allowing the QoS IE to besent from the MME/S4-SGSN will eventually lead to a risk of allowing QoSupdates from MME/S4-SGSN which adds a considerable amount of complexityin the PGW.

SUMMARY

It is therefore an object of the present invention to address some ofthese problems. This is done by introducing a new indicator in a GPRStunneling version 2 protocol that is used to signal:

1. From source MME to the target S4-SGSN in case of EUTRAN toUTRAN/GERAN handover over the S3 interface, and

2. from source S4-SGSN to target S4-SGSN in case of SGSN relocation inUTRAN/GERAN over a S16 interface, and

3. from S4-SGSN to target SGW over an S4 interface, and

4. from SGW to PGW over an S5/S8 interface, that the EPS bearer relatedvoice has been handed over to the CS domain, e.g. according to theprinciples of SRVCC.

The solution according to the present invention is provided in a numberof aspects, in which a first is a method in a telecommunications networkconnecting user equipment, i.e. UE, communicating wirelessly with thenetwork. The method comprises steps of detecting that handover isrequired from a packet switched (PS) based network to a circuit switched(CS) based network, initiating in a source mobility management node asingle radio voice call continuity (SVRCC), initiating a PC to CShandover for non-voice components with information about voice relatedparameters and a PS to CS handover indicator, executing hand over,sending an update request to a serving gateway (SGW) from a targetmobility management node with non-voice related parameters and the PS toCS handover indicator, forwarding the update request from the SGW to apacket data network gateway (PGW), receiving the update request in thePGW, detecting the PS to CS handover indicator, and handling in the PGWthe PS to CS handover indicator.

The step of handling in the PGW may comprise waiting for a decision froma policy and charging rules function node (PCRF). The method may furthercomprise steps of receiving a decision from the PCRF to removeauthorization for an associated voice call service and freeing up bearerresources.

The step of handling in the PGW may comprise transmitting the PS to CShandover indicator information to a policy and charging rules functionnode. The source and/or target mobility management node may be one ofMME or SGSN.

The handover may be required from an E-UTRAN or from a UTRAN-HSPA basednetwork to a UTRAN/GERAN based network.

The step of initiating handover may be executed with a forwardrelocation request message. The method may further comprise steps ofreceiving in the PGW an indication that voice services are not anylonger authorized in the PS domain and freeing up resources in the PGW.

Another aspect of the present invention is provided, a node in atelecommunications network connecting user equipment (UE) wirelessly tothe network. The node may be configured to receive an update requestfrom a serving gateway (SGW) in relation to a single radio voice callcontinuity event (SRVCC) when handover is required from a packetswitched (PS) based network to a circuit switched (CS) based network,wherein the node may further be configured to detect a PS to CS handoverindicator related to voice components in the update request, and tohandle the PS to CS handover indicator.

The node may be configured to wait for a decision from a policy andcharging rules function node (PCRF) or configured to transmitinformation related to the PS to CS handover indicator to the PCRF. Thenode may be a packet data network gateway node.

Yet another aspect of the present invention is provided, a system in atelecommunications network. The system comprising a packet data networkgateway (PGW), a mobility management node (MME), and a serving gatewaynode (SGW). The MME may be arranged to obtain a handover requiredmessage for a user equipment, to initiate bearer splitting of voice andnon-voice components, and to send an update request to the SGW which isarranged to forward the update request to the PGW, wherein the updaterequest comprises both non-voice related parameters and a packetswitched to circuit switched handover indicator.

Advantages of the present invention are that it efficiently resolves arace condition for IMS that may result in unnecessary loss of ongoingphone calls. The indicator as such is advantageously mandatory in orderto support SRVCC in the EPC. This means that support may be needed inall of the following network nodes: MME, S4-SGSN, SGW, PGW andpotentially in the PCRF.

Introducing a separate indicator instead of re-using the preservationindication as part of the QoS IE will avoid future problems withintroducing QoS updates from the MME/S4-SGSG which would add aconsiderable amount of complexity in the PGW. It also avoidsintroduction of a complete preservation mechanism in EPS, which wouldhave caused a large amount of confusion in GPRS.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in a non-limiting wayand in more detail with reference to exemplary embodiments illustratedin the enclosed drawings, in which:

FIG. 1 illustrates schematically a network according to the presentinvention;

FIG. 2 illustrates schematically in a sequence diagram interactionbetween network nodes according to the present invention;

FIG. 3 illustrates schematically in a block diagram a method accordingto the present invention; and

FIG. 4 illustrates schematically in a block diagram a device accordingto the present invention.

DETAILED DESCRIPTION

In FIG. 1 reference numeral 100 generally indicate a telecommunicationscore network according to the present invention. The network comprises acore network comprising a mobility management node, a gateway, and apolicy and charging rules function node. The present invention addressesissues during handover between two different radio access technologies.In the example shown in FIG. 1, a handover from a 3GPP EUTRAN basednetwork to a 3GPP UTRAN/GERAN based network is shown. In the networkexample of FIG. 1 a UE 101 is attached to a EUTRAN based access network102 (AGW) which provides access to the core network 100. In the corenetwork a mobility management entity (MME) 104 provides mobilityhandling for the UE and takes part in the transfer of for instance voicesession to the UTRAN/GERAN based access network 103. Furthermore, thenetwork comprises an SGSN 105, serving gateway (SGW) 107, packet datanetwork gateway (PGW) 108, Policy and charging rules function node(PCRF) 109, and a mobile switching center (MSC) 106 handling voicerelated issues in the UTRAN/GERAN network part. The network is alsoconnected to an IP multimedia subsystem (IMS) 110.

The present invention handles a situation where handover of voice beareris to be performed from a network with both voice and non-voice bearercapability to a network primarily handling voice bearers. The presentinvention is applicable both at Single Radio Voice Call Continuity(SRVCC) from EUTRAN to UTRAN/GERAN and at SRVCC from UTRAN (HSPA) toUTRAN/GERAN.

With reference to FIG. 2, an example of sequence flow of a SVRCC fromEUTRAN to UTRAN/GERAN with dual transfer mode (DTM) and Packet switched(PS) handover (HO) support is shown. Even though SVRCC from EUTRAN toUTRAN is exemplified it should be noted that the present invention isalso valid for the case of SRVC from UTRAN with High Speed Packet Access(HSPA) to UTRAN/GERAN with DTM HO when an S4 based SGSN is used.

FIG. 2 illustrates in a sequence diagram a handover situation fromEUTRAN to UTRAN/GERAN with DTM HO. The UE 101 sends 201 one or severalmeasurement reports to the source network 102 (e.g. EUTRAN) indicatinglink quality and depending on the content of the reports the sourcenetwork (NW) takes a handover decision 202 and decides to trigger anSRVCC handover to UTRAN/GERAN. The source NW sends 203 a handover (HO)required message comprising information about target ID, generic sourceto target transparent container, optionally additional parameters, andan indication that this is for CS+PS handover to the MME 104 which inturn takes a decision 204 of bearer splitting, i.e. separating voice anddata connections, and initiates the bearers relocation towards an MSCserver and SGSN. The MME (source) sends 205 a message indicating packetswitched (PS) to Circuit Switched (CS) handover request to the MSCserver/Media gateway (MGW) 229. The MSC/MGW server sends 206 a prepareHO message to the target MSC 106. The target MSC sends 207 arelocation/handover request to a target Radio Area network (RN) 103. Inone embodiment the steps 205-207 may comprise in more detail that theSource MME initiates the PS-CS handover procedure for the voice bearerby sending a SRVCC PS to CS Request (Target ID, STN-SR, MSISDN, Sourceto Target Transparent Container, MM Context) message to the MSC Server.The MSC server is selected based on the Target ID received in theHandover Required message. The Target ID is also used by the MSC Serverto identify the target RNS/BSS. The message includes informationrelevant to the CS domain only. MME received STN-SR and MSISDN from theHSS as part of the subscription profile downloaded during the EUTRANattach procedure. MM Context contains security related information. CSsecurity key is derived by the MME from the EUTRAN/EPS domain key. TheCS Security key is sent in the MM Context. MSC Server interworks thePS-CS handover request with a CS inter-MSC handover request by sending aPrepare Handover Request message to the target MSC. Target MSC requestsresource allocation for the CS relocation by sending the RelocationRequest/Handover Request (additional Source to Target TransparentContainer) message to the target RNS/BSS.

The MME sends 208 a forward relocation request to the target SGSN 105.This forward relocation request comprises EPS bearer information notonly for non-voice components but also information on the voice relatedbearers with an indicator that they have been handed over to the CSdomain; e.g. the request may comprise source to target transparentcontainer, MM context, and PDN connections. This indicates that thetarget S4-SGSN does not need to try to establish Radio Access Bearers(RAB) for these bearers. The target SGSN exchanges 209Relocation/handover request and request acknowledgements with the targetradio network (RNS)/Base station (BSS) subsystem 103. The target SGSNwill then send 210 a forward relocation response to the MME. The targetRN sends 211 a relocation/handover request acknowledgement message tothe target MSC also. In response to this the target MSC and MSC serverexchanges 212 prepare handover response and establishment circuitmessages. The MSC server then sends 213 a message for initiation ofsession transfer (STN-SR) to the IP multimedia subsystem (IMS) and theMSC server also sends 216 a PS to CS response message to the MME.Meanwhile, the IMS will handle session transfer issues and update remoteleg 214 and release of IMS access leg 215. The MME will send 217 ahandover command to the source network which in turn sends 218 ahandover from EUTRAN command to the UE whereby the UE tunes 219 into theUTRAN/GERAN network.

Handover detection is then performed 220 in the UE, Source NW, MME, MSCserver, target MSC, SGSN and target RN.

The target RN sends 221 a relocation/handover complete message to thetarget MSC and the target MSC exchanges 222 speech enabled services(SES) handover messages with the MSC server. The MSC server exchanges223 PS to CS complete acknowledgement messages with the MME and sends224 an update location message to appropriate Home subscriber server(HSS) or home location register (HLR) 225. The target RN sends 226 arelocation/handover complete message to the SGSN which in turn exchanges227 forward relocation complete acknowledgement messages with the MME.Finally, the SGSN, SGW, PGW, and PCRF exchanges 228 update bearermessages which comprise information on voice related bearer togetherwith the handover indicator which indicates that the voice bearers havebeen handed over to the CS domain. The update bearer messages originatefrom the SGSN and sent to the SGW but may be forwarded to the PGW andthe PCRF. Target SGSN may update bearers with SGW; the PS to CS HOindicator is set for the voice bearer(s).

It should be noted, that in case of Gn/Gp SGSN, the PS to CS indicatormay not be signalled for the voice bearer(s). Instead a multiple bitrate (MBR) parameter may be set equal to zero to indicate HO to the CSdomain.

The PGW is thus informed of the situation and is aware of which bearersthat have been handed over to the CS domain. Either the PGW may directlyforward this information to the PCRF, comprising the handover indicator,or it may await the PCRF decision to remove authorization for theassociated voice call service over Gx. When this happens the PGW willfree up potential resources internally associated to the bearers.

With reference to FIG. 3, the following method steps show a generalexample of the present invention:

301. A UE with both and non-voice bearers is currently in EUTRAN or inUTRAN (HSPA) when the serving networks detect that a handover (HO) toUTRAN/GERAN is required.

302. Since the target network (NW) relies on legacy circuit switched(CS) domain for voice the source mobility management entity (MME) orsource S4-SGSN initiates SRVCC to target MSC server for the voice bearer(component).

303. In parallel an ordinary PS to PS handover is initiated for thenon-voice bearers. However, the voice bearers are included but the PS toCS handover indicator is set.

304. The handover is then executed, i.e. the UE switch to UTRAN/GERAN.

305. The target S4-SGSN sends an update request to the SGW comprisingthe successfully established non-voice bearers and PS to CS indicatorset for the voice bearers.

306. The SGW forwards the message to the PGW.

307. The PGW receives the request. The PGW learns by means of the PS toCS indicator that the voice bearers have been handed over to the CSdomain. Either the PGW awaits the PCRF decision or reports theinformation to the PCRF.

FIG. 4 shows a node 400 in the telecommunications network infrastructurearranged to handle parts of the method according to the presentinvention. The node comprises at least one processor 401, at least onememory unit 402, and at least one communication interface 403. Theprocessor may comprise any suitable type of processing unit, such as forinstance a microprocessor, digital signal processor (DSP), applicationspecific integrated circuit (ASIC), or field programmable gate arrayFPGA). The processor is arranged to execute instruction sets, such assoftware or hardware instruction sets. For instance softwareinstructions may be stored in the memory unit which may comprise avolatile or non-volatile memory type. The memory unit may comprise anysuitable memory being of computer readable storage type, e.g. hard disk,RAM, EPROM, Flash, and so on. The memory may also be utilized forintermediate storage of statistics, software data, and so on.Furthermore, the processor is arranged to utilize the communicationinterface to communicate with other nodes in the network. Thecommunication interface is preferably of a packet type and preferablyutilizes a common standardised protocol such as for instance Ethernet.All nodes in the infrastructure network comprise similar hardwarearchitecture. In the present invention, the MME, the SGSN and the PGWuse the handover indicator in the handover process.

The present invention may be utilized in a number of differentsituations/network configurations. The handover indicator is used inGPRS tunneling version 2 protocol and may be applicable to at least oneof the following situations and the indicator is used to signal:

1. From source MME to the target S4-SGSN in case of EUTRAN toUTRAN/GERAN handover over the S3 interface, and

2. from source S4-SGSN to target S4-SGSN in case of SGSN relocation inUTRAN/GERAN over a S16 interface, and

3. from S4-SGSN to target SGW over an S4 interface, and

4. from SGW to PGW over an S5/S8 interface,

that the EPS bearer related voice has been handed over to the CS domain,e.g. according to the principles of SRVCC.

Based on this information either the PGW may take local action or itcould forward this information to the PCRF.

In case of local action the PGW may e.g. avoid to inform the PCRF thatthe bearer resources related to the voice bearer bas been lost (case 1).Alternatively, the

PGW may inform the PCRF that the bearer has been lost, but hen theindicator is passed along over the Gx interface (case 2). For case 2 thePCRF may respond to the PGW with an updated policy decision.

Eventually, the PCRF will receive information over the Rx interface thatthe voice is no longer authorized in the PS domain.

For case 1, the PCRF will then notify the PGW over Gx interface that theservice is no longer authorized. The PGW may then free up resourcesinternally.

For case 2, the PCRF will correlate the information received over Rxinterface with the information received over Gx interface. If necessarythe PCRF will update its policy decision towards the PGW.

In both cases the race condition described in the background section isavoided.

It should be noted that the word “comprising” does not exclude thepresence of other elements or steps than those listed and the words “a”or “an” preceding an element do not exclude the presence of a pluralityof such elements. It should further be noted that any reference signs donot limit the scope of the claims, that the invention may be at least inpart implemented by means of both hardware and software, and thatseveral “means” or “units” may be represented by the same item ofhardware.

The above mentioned and described embodiments are only given as examplesand should not be limiting to the present invention. Other solutions,uses, objectives, and functions within the scope of the invention asclaimed in the below described patent claims should be apparent for theperson skilled in the art.

ABBREVIATIONS

3GPP 3rd Generation Partnership Project

AGW Access Gateway

APN Access Point Name

ARP Allocation and Retention Priority

BSC Base station controller

BSS Base station subsystem

DTM dual transfer mode

eNB evolved Node B

EPC Evolved packet core

EPS Evolved Packet System

E-UTRAN Evolved UTRAN

GBR Guaranteed bit rate

GCS Gateway Control Session

GGSN Gateway GPRS support node

GPRS General Packet Radio Service

GTP GPRS Tunneling Protocol

GW Gateway

HI Handoff Indicator

HLR Home location register

HO Handover/Handoff

HSPA High Speed Packet Access

ICS IP-CAN Session

IE Information element

IMS IP multimedia Subsystem

IP Internet Protocol

IP-CAN IP-Connectivity Access Network

MBR Multiple bit rate

MGW Media Gateway

MME Mobility management entity

MSC Mobile switching center

PBA Proxy binding Acknowledgement

PBU Proxy binding Update

PCC Policy and Charging Control

PCEF Policy and Charging Enforcement Function

PCRF Policy and charging rules function

PDN GW Packet Data Network Gateway

PDN Packet Data Network

PDP Packet data protocol

PMIP Proxy Mobile IP

QoS Quality of Service

RAT Radio access technology

RNC Radio network controller

RNS Radio Network Subsystem

SAE System Architecture Evolution

SGSN Serving GPRS support node

SRVCC Single Radio Voice Call Continuity

UE User Equipment

UMTS Universal Mobile telecommunications system

UTRAN Universal terrestrial radio access network

1. A node to be used in a telecommunications network, thetelecommunications network connecting user equipment, UE, wirelessly tothe telecommunications network, wherein the node is configured toreceive an update message in relation to a single radio voice callcontinuity event, SRVCC, when handover is required from a packetswitched, PS, based network to a circuit switched, CS, based network,which update message comprises a PS to CS indicator indicating thatvoice bearers have been handed over to the CS domain.
 2. The node ofclaim 1 wherein the node is configured to receive the update messagefrom a target mobility management node.
 3. The node of claim 2 whereinthe update message includes both the PS to CS indicator and non-voicerelated parameters.
 4. The node of claim 2 wherein the target mobilitymanagement node is a mobility management entity.
 5. The node of claim 2wherein the target mobility management node is a serving general packetradio service support node, SGSN.
 6. The node of claim 1 wherein thenode is a serving gateway.
 7. The node of claim 1 wherein the handoveris required from an evolved universal terrestrial radio access network,E-UTRAN, based network to a universal terrestrial radio accessnetwork/global system for mobile communications enhanced data rates forglobal evolution radio access network, UTRAN/GERAN, based network. 8.The node of claim 1 wherein the handover is required from a universalterrestrial radio access network-high speed packet access, UTRAN-HSPA,based network to a universal terrestrial radio access network/globalsystem for mobile communications enhanced data rates for globalevolution radio access network, UTRAN/GERAN, based network.
 9. A methodof operation of a node in a telecommunications network connecting userequipment, UE, wirelessly to the telecommunications network, comprising:receiving, by the node, an update message in relation to a single radiovoice call continuity event, SRVCC, when handover is required from apacket switched, PS, based network to a circuit switched, CS, basednetwork, which update message comprises a PS to CS indicator indicatingthat voice bearers have been handed over to the CS domain.
 10. Themethod of claim 9 wherein receiving the update message comprisesreceiving the update message from a target mobility management node. 11.The method of claim 10 wherein the update message includes both the PSto CS indicator and non-voice related parameters.
 12. The method ofclaim 10 wherein the target mobility management node is a mobilitymanagement entity.
 13. The method of claim 10 wherein the targetmobility management node is a serving general packet radio servicesupport node, SGSN.
 14. The method of claim 9 wherein the node is aserving gateway.
 15. The method of claim 9 wherein the handover isrequired from an evolved universal terrestrial radio access network,E-UTRAN, based network to a universal terrestrial radio accessnetwork/global system for mobile communications enhanced data rates forglobal evolution radio access network, UTRAN/GERAN, based network. 16.The method of claim 9 wherein the handover is required from a universalterrestrial radio access network-high speed packet access, UTRAN-HSPA,based network to a universal terrestrial radio access network/globalsystem for mobile communications enhanced data rates for globalevolution radio access network, UTRAN/GERAN, based network.